Highly porous cellulosic paper products have a variety of personal, commercial, and industrial uses. For instance, the porosity of the paper makes the paper useful as a filter for the purification of fluid streams. Common examples of such filters are conventional air filters, water filters, and oil filters.
Cellulose fibers that form the basis of the highly porous papers are specially treated in order to stiffen the fibers and give the individual fibers a rod shape, in contrast to the ribbon-shaped fibers used in traditional paper making. The stiffened, rod-shaped fibers allow for the production of low density paper products that have acceptable structural properties.
One method of stiffening the cellulose fibers is “mercerization”, a method of treating the cellulosic fibers with concentrated caustic solutions in order to alter the morphology of the fiber structure by changing the crystallinity of the cellulose fiber from cellulose I to cellulose II. Mercerization may be accomplished by using a cold caustic treatment process. Cold caustic treatments are generally performed by mixing a low consistency cellulose slurry with an alkaline solution, typically a sodium hydroxide solution, and allowing the mixture to steep, or react, for a suitable amount of time. Mercerization transforms the ribbon-like cross section of the raw cellulosic fibers into a round shape, thereby increasing the stiffness of the fibers. The steeped cellulose fiber is subsequently washed to remove the caustic solution and then may be subjected to downstream processing.
Filter paper is typically produced by depositing mercerized fiber pulp upon a web, as in ordinary paper production, under conditions that result in the desired porosity of finished paper product.
Prior to use, sheets of filter paper are often folded or crimped in order to increase the available surface area of the filter incident to the volume of fluid being filtered. For instance, filter papers are commonly crimped in alternating directions along the length of the paper in order to form the paper into an accordion-shaped filter, which is then disposed within a filter housing.
Because the filter paper is constructed of a low-density arrangement of stiff cellulose fibers, folding and crimping of the paper tends to cause significant dusting. Dusting is the unintended separation of cellulose particles from a paper material upon manipulation of the paper. Dusting is particularly problematic during filter formation due to the large number of crimps and bends made during formation of a filter and because the stiffened fibers of the mercerized filter paper tend to flake off easily when the paper is manipulated.
Previous attempts to minimize dusting have involved application of adhesive compounds to the finished filter paper in order to fix the cellulose fibers in place. Although application of adhesive to the paper does reduce dusting, the adhesive tends to adversely reduce the porosity of the filter. Thus, there remains a need in the art to develop a method to reduce dusting in highly porous cellulosic paper products without significantly reducing the porosity of the filter paper.